In case you are not familiar with ODB, it is an object-relational mapping (ORM) system for C++. It allows you to persist C++ objects to a relational database without having to deal with tables, columns, or SQL, and manually writing any of the mapping code.

For the complete list of changes, see the official ODB 1.7.0 announcement. While there are quite a few cool new features in this release, the biggest one, no doubt, is support for the Oracle database. As usual, below I am going to examine this and other notable new features in more detail.

Oracle support

Support for Oracle is provided by the libodb-oracle runtime library. All the standard ODB functionality is available to you when using Oracle, including support for containers, object relationships, queries, date-time types in the Boost and Qt profiles, etc. In other words, this is complete, first-class support, similar to that provided for PostgreSQL, MySQL, and SQLite. There are a few limitations, however, most of which are imposed by the underlying OCI API. Those are discussed in Chapter 16, “Oracle Database” in the ODB Manual.

For connection management in Oracle, ODB provides two standard connection factories (you can also provide your own if so desired): new_conection_factory, and conection_pool_factory.

The new connection factory creates a new connection whenever one is requested. Once the connection is no longer needed, it is closed.

The connection pool factory maintains a pool of connections and you can specify the min and max connection counts for each pool created. This factory is the default choice when creating a database instance.

If you had any prior experience with ODB, you are probably aware that one of our primary goals is high performance and low overhead. For that we use native database APIs and all the available performance-enhancing features (e.g., prepared statements). We also cache connections, statements, and even memory buffers extensively. The Oracle runtime is no exception in this regard. The question you are probably asking now is how does it stack up, performance-wise, against other databases that we support.

Unfortunately, Oracle Corporation doesn’t allow anyone to publish any hard performance numbers about its database. This is explicitly prohibited in the license, even for Oracle Express. So all I can do in this situation is give to you some general indications and make it easy to run a few benchmarks for yourself.

The first benchmark that we normally run is the one from the Performance of ODB vs C# ORMs post. Essentially we are measuring how fast we can load an object with a couple of dozen members from the database. In the previous announcements I mentioned that it takes ODB with PostgreSQL 9.0.4 27ms per 500 iterations (54μs per object), MySQL 5.1.49 — 24ms (48μs per object), and SQLite 3.7.5 — 7ms (14μs per object). Oracle 11.2 results are on par with PostgreSQL and MySQL. To get the exact numbers, feel free to download the benchmark source code and give it try. The accompanying README file has more information on how to build and run the test.

To measure the concurrent access performance we use an update-heavy, highly-contentious multi-threaded test in the ODB test suite, the kind you run to make sure things work properly in multi-threaded applications (see odb-tests/common/threads if you are interested in details). It normally pushes my 2-CPU, 8-core Xeon E5520 machine, which runs the database server, close to 100% CPU utilization. As you may remember, PostgreSQL 9.0.4 was the star of this benchmark, beating both MySQL 5.1.49 with the InnoDB backend and SQLite 3.7.5 by a significant margin (12s vs 186s and 48s, respectively). Again, the licensing terms prevent me from revealing any concrete numbers. But let me say that Oracle performance in this test is commensurable with the amount of money one has to pay for it. In particular, the higher the edition of Oracle you are using and thus the more CPUs it can use, the better the performance. Interestingly, even the free Express edition, which is limited to 1 CPU and 1GB or RAM, outperforms MySQL that has 8 cores and 12GB of RAM available to it.

Let me also note that these numbers should be taken as indications only. It is futile to try to extrapolate some benchmark results to your application when it comes to databases. The only reliable approach is to create a custom test that mimics your application’s data, concurrency, and access patterns. Luckily, with ODB, creating such a test is a very easy job. You can use the above-mentioned benchmark source code as a starting point.

Optimistic concurrency

Another big feature in this release is support for optimistic concurrency. To make a persistent class “optimistic”, all we have to do is declare it as such and add an integer data member that will store the object version. For example:

Now, whenever we update the state of the person object in the database, ODB will check if it has in the meantime been modified by someone else and throw the odb::object_changed exception if that’s the case. Chapter 11, “Optimistic Concurrency” in the ODB Manual has more information as well as a nice overview of the optimistic concurrency concept, if you are not familiar with the idea.

SQL statement execution tracing

Quite a few ODB users asked us to provide a way to trace SQL statements that are being executed as a result of database operations. While the database can normally provide a log of SQL statements, being able to do this from the application has a number of advantages. In particular, this way we can trace SQL statements only for a specific transaction, or even a specific set of ODB operations.

ODB allows us to specify a tracer on the database, connection, and transaction levels. We can either provide our own implementation of the tracer interface (odb::tracer), or we can pass the built-in odb::stderr_tracer implementation that prints the statements to STDERR. This example shows how we can trace all the statements executed on a specific database:

ODB automatically treats const data members as read-only. It is also possible to declare the whole persistent class as read-only.

Read-only members are primarily useful when dealing with asynchronous changes to the state of a data member in the database which should not be overwritten. In other cases, where the state of a data member never changes, declaring such a member read-only allows ODB to perform more efficient object updates. In such cases, however, it is conceptually more correct to declare such a data member as const rather than as read-only. So, the above example is probably better re-written like this:

#pragma db object
class person
{
...
const std::string name_;
};

Persistent classes without object ids

Up until this release every persistent class had to have an object id (which translates to PRIMARY KEY in the database). But as it is sometimes desirable to have a table without the primary key, so it is sometimes desirable to have an object without an object id. With this release ODB adds support for persistent classes without object identifiers. Note, however, that such classes have to be explicitly declared as not having an object id:

#pragma db object id()
class person
{
...
};

Such classes also have limited functionality. In particular, they cannot be loaded with the database::load() or database::find() functions, updated with the database::update() function, or deleted with the database::erase() function. To load and delete objects without ids you can use the database::query() and database::erase_query() functions, respectively. There is no way to update such objects except by using native SQL statements.

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on Wednesday, December 7th, 2011 at 5:22 am and is filed under ORM, C++.
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